This was written a year ago after a few weeks consideration, and a half a lifetime of playing, as far as HOW a Magic card should be made.
Rules were needed.
Codified.
I probably missed something but this is it.

CREATURES
Priced for Common rarity.
Uncommons seem to apply a reduction to mana cost of 1 . Rares vary but tend to be -2 mana cost in the same way.
Round fractions down after adding all cost additions
The "game breaks" after about 6-7 mana so costs for creatures at that point are difficult to factor

SIZE
You may add 1 point to Toughness by subtracting 1 point from Power repeatedly, and the other way around
Size Points are distributed between Power and Toughness
There is no minimum Toughness but a 0 Toughness creature dies as soon as it enters the battlefield unless another effect boosts it

0 mana
1 Size Point
0/1

1 mana
2 Size Points
1/1

2 mana
3 Size Points
1/2
2/1 (white or green only)

3 mana
4 Size Points
2/2

4 mana
5 Size Points
2/3
3/2 (white or green only)

5 mana
6 Size Points
3/3

6 mana
7 Size Points
3/4
4/3

3 colored mana
4/4 (green only)

7 mana
9 Size Points
4/5
5/4

8 mana
10 Size Points
5/5
6/4

9 mana
6/6 ?
8/8 (green only)
At this point sizes vary wildly, but runs in favor of green

ABILITIES
Limit 3 abilities total
Limit 2 flaws total
Abilities may be taken from another color list at the expense of 1 extra mana added to total cost
By adding an activation cost of +1 mana for an ability (to use until end of turn) the total ability cost is reduced by 1

To create a spell, the default spell template is a sorcery for 0 mana that does nothing. Add abilities onto that.

In general to make an activated ability of a permanent add 2 mana to the activation cost, which can be reduced with flaws

Artifacts may access abilities of any color. The drawback is that they are easier to destroy.

Any color:

1/2 mana
Living Weapon (card is an artifact that plays equipped to a 0/0 Germ token, can equip to other creatures)

1 mana
Card is an instant rather than a sorcery
Bushido X (X is equal to the extra mana cost)
Cycling 2 (pay 2 mana to cycle)
All mana symbols are hybrid with a second color
Protection from an opposing color or same color
Shadow
Annihilator 2 (only applied if total cost is at least 8)
All colored symbols of the card are Phyrexian mana (pay the mana cost or two life per symbol)
Power and toughness are equal to tribal or color count
Put a -1/-1 counter on target creature when this card is played
The number of this specific card in a deck is unlimited
The card is shuffled into your library if enters a graveyard
If an ability destroys one type of permanent, add another type within the limits of what that color is capable of (white: artifacts and enchantments, green: anything but creatures, black: creatures)

2 mana
Cascade
One ability affects all creatures you control, or all creatures your opponent controls
Put a 1/1 creature token into play when this card dies, taps, is played, or when a card type is played such as land, instant, or artifact
When this card is played or enters a graveyard search for a basic land and put it in your hand
Retrace

3 mana
Indestructible
Put a 1/1 creature token into play of your choice of tribal and color when this card dies, taps, is played, or when a card type is played such as land, instant, or artifact
When this card is played or enters a graveyard search for a basic land and put it into play tapped

4 mana
Card reduces the mana cost of all spells of a single color or card type by 1 mana
Proliferate (requires tapping or occurs when card is played)
When this card is played or enters a graveyard search for a basic land and put it into play

5 mana
Whenever a card type you control or play deals damage increase that damage by 2
When this card is played or enters a graveyard search for a land and put it into play

2 mana
Fights when enters the battlefield
Target creature gets +3/+3 until end of turn, or more if this ability is costed higher
+1/+1 until end of turn from a permanent source multiple times
+X/+X until end of turn when X is tribal count or color count
Put a +1/+1 counter on target creature as a spell
Regenerate target creature (pay 1 colored mana)
Put a 1/1 creature token onto the battlefield as a spell

3 mana
Destroy target artifact or enchantment when this card is played

4 mana
Destroy target land when this card is played
Put a 1/1 creature token onto the battlefield as an ability from a permanent
Put a +1/+1 counter on target creature as an ability from a permanent

White abilities:

1 mana
Battlecry
Exalted
Flanking
Lifelink
Persist
Gain life equal to the creature's Toughness when it enters the battlefield
Plainswalk
Vigilance
First Strike
Banding
Convoke
Destroy target enchantment when this card is played
Destroy target artifact when this card is played

2 mana
Doublestrike
Gain 3 life when this card is played or destroyed, +2 life for each additional mana
Prevent 3 damage to a creature or player when this card is played

8 mana
Destroy all creatures when this card is played

11 mana
Destroy all nonland permanents when this card is played

Blue abilities:

1 mana
Draw a card, either by paying 5 mana, tapping, or being played
Islandwalk
Flying
Scry 3 when the card is played
Scry 1 when card type or tribal is played
Shroud
Flash
Tap target creature
Untap target creature
Untap target artifact

2 mana
Place the top 5 cards of target player's library into their graveyard
Unsummon a creature
Force target creature an opponent controls to attack

3 mana
Unsummon a nonland permanent
Force all creatures an opponent controls to attack
Tap to deal 1 damage to target creature or player

4 mana
Counter target spell when this card is played (must be paired with instant speed)
Unsummon a permanent
Draw 2 cards as the card is played

2 mana
Target creature gets -2/-2 until end of turn, or more if this ability is costed higher
Add 3 black mana to your mana pool when this card is played

4 mana
Destroy target nonblack or tapped creature when this card is played or taps
Put a black 0/1 creature token onto the battlefield

5 mana
Destroy target creature when this card is played or taps

8 mana
Destroy all creatures when this card is played

11 mana
Destroy all creatures you don't control when this card is played

Red abilities:

1 mana
Devour 1
Haste
Intimidate
Deals damage equal to Power to another creature when enters the battlefield
Flanking
Provoke
First Strike
A creature dealt damage by this card can't regenerate until end of turn
+1 Power until end of turn

2 mana
Doublestrike
Deal 3 damage to target creature or player when this card is played
Target creature gets +4/-4 to a minimum of 1 Toughness until end of turn
Destroy target artifact when this card is played

3 mana
Tap to deal 1 damage to target creature or player

4 mana
Destroy target land when this card is played

FLAWS
Worth a subtraction of 1 mana from total mana cost

Legendary
Defender
Can't block
Enters battlefield tapped
Ability requires tapping if not already
Deals damage or lose life equal to Power to controller when enters battlefield
Champion a creature type shared with the card (worth 3 reduced)
Sacrifice this card for effect
Sacrifice a land for effect
Sacrifice another creature for effect
Second color: At least 1 colored mana symbol is of an additional color
Echo (pay mana cost again during next upkeep or sacrifice it)
Ability activation costs 1 additional mana
Ability activation costs discarding a card
Ability only activates when creature attacks/blocks
Must attack each turn
Must block each turn
Ability requires Threshold
Mana generated by this creature is colorless
Ability only affects, or depends on such as X mana generated, tribal or color of the same type
Mana generated by the card can only be spent on cards of a single type such as creatures or artifacts
The creature is an artifact as well as colored
The creature is an enchantment
Ability only occurs when blocked or blocking or when dealing combat damage
Ability only affects tapped, blocking, attacking, or blocked targets
Controller of target spell or permanent that was destroyed or countered gainds 4 life, draws a card, searches for a basic land and puts it into play tapped, or puts a 3/3 Beast creature token onto the battlefield (worth 2 reduced)
Taps when trigger occurs_________________My fiction blog and novelette